Much of our present understanding of the neuronal and synaptic organization of the oculomotor system has evolved from morphological, physiological, and clinicopathological studies of the neurones, nuclei, and pathways that are related to various types of eye movement. The correlation of the neuronal activity of premotor neurones with the activity of both the oculomotor motoneurones and the extracular muscles suggests synaptic relationships that underly neuronal interactions in the oculomotor system, particularly in the vestibulo-ocular reflex. The specific aims of the proposed studies are directed toward delineating similarities and/or differences in the patterns of synaptic connectivity of second-order vestibular neurones with motoneurones in the oculomotor, trochlear, and abducens nuclei. Physiologically-identified second-order vestibular neurones or axons will be identified electrophysiologically and stained by intracellular injection of horseradish peroxidase, and their axonal arborizations will be reconstructed by light and electron microscopy to determine the modes and patterns of synaptic connections with different populations of motoneurons. In addition, the postnatal development of abducens motoneurones and internuclear neurones and of the lateral rectus muscle will be examined by electron microscopy in relation to the synaptic inputs to the neurones and the sequential differentiation of different muscle fiber types. The proposed studies are consistent with the long-term goal of elucidating neuronal and synaptic relationships in the oculomotor system underlying gaze and their relationship to disorders of ocular motility.